This invention relates to reducing heat buildup and improving window defrosting/defogging performance in a motor vehicle and more particularly to doing so by the structure of the windshield.
Windshields of vehicles such as automobiles are conventionally of laminated glass comprising thermoplastic interlayer, such as a plasticized polyvinyl butyral or polyurethane sheeting, sandwiched between two sheets of glass. These assemblies are mounted in openings extending across the upper front ends of the vehicle bodies and, in modern designs, are inclined backwardly at a substantial angle from the vertical thereby admitting sunlight into large areas of the vehicle interior. The interlayer conventionally has an integral marginal band of a light-absorbing dye above the driver's direct line of vision through the windshield to reduce sunlight glare. This band is graduated in intensity, being greatest near the upper periphery of the interlayer when in place in a windshield and gradually diminishing to an almost imperceptible level at the lower edge of the band.
Solar screening films have been incorporated into these assemblies to reduce solar radiation influx while maintaining high luminous or visible light transmission, thus reducing temperature buildup within the vehicle from exposure to the sun's rays without adversely affecting visibility through the windshield. Such films are called interference or induced transmission filters and comprise a multi-layer coating of at least one layer of a reflective metal sandwiched between reflection-suppressing dielectric layers. Representative structures for motor vehicle windshields are disclosed in International Publication No. WO 88/01230 and U.S. Pat. No. 4,799,745.
For safety purposes windshields containing solar screening films must still comply with a federally-mandated 70% minimum luminous transmission level in the area of the driver's direct field of view, and for convenience should desirably still have the capability of minimizing glare. Moreover, there is a recent trend toward extending windshields back into the roof panel to provide a more open, spacious feeling to the occupants. The level of solar flux against an overhead windshield extension in the roof area, which is orthogonal to the sun's rays, is essentially maximum. However, absorptive solar rejection, by a dyed band or by heat absorbing glass has little or no effect on control of the internal equilibrium temperature of a vehicle parked in direct sunlight, because absorbed heat contributes substantially to increasing the interior vehicle temperature. Furthermore, though reducing luminous transmission in the visible region, a dyed gradient band provides no attenuation in the near infra-red region (700-3000 nm).
Furthermore, windshields having electrical defrosting or defogging capability are desirable and this is conveniently achieved through use of the metal layer in the interference filter as a conductor, as is disclosed in U.S. Pat. Nos. 4,782,216 and 4,786,783. While these systems are generally acceptable, electrical conductivity is constant throughout the windshield and, though also desirable, there is no capability of preferentially directing electric current flow to selected regions of the windshield through which the vehicle operator looks to maximize conductivity and wherefore minimize defrost/defog time.